This invention relates to a packaging method using lead-free solder alloy metal the toxicity of which is minor, a soldering system, and a mounting structure in which the packaging method and soldering system are used. This lead-free solder alloy metal can be applied to connect an electronic device to a circuit board of an organic board and the like, and is used as a substitution of Sn-37Pb (Unit: Mass %) which is used for a soldering at about 220xc2x0 C.
A conventional method to solder a device to a circuit board made of an organic material and the like of electric appliances comprises a reflow-soldering step in which hot air is blown against the circuit board, solder paste printed on electrodes is molten, and thus, a surface mount device is soldered, and a flow-soldering step in which a molten solder jet is caused to come into contact with a circuit board, and thus, some surface mount devices such as a insertion mount device, a chip device and the like are soldered.
And, this soldering method is called a packaging method.
Now, it has been requested to use lead-free solder alloy metal the toxicity of which is minor for both the solder paste used in the reflow-soldering step and the molten solder jet used in the flow-solder step.
As the prior arts related to this packaging method using lead-free solder, Japanese Patent Laid-open No. 10-166178 (Prior Art 1), No. 11-179586 (Prior Art 2), No. 11-221694 (Prior Art 3), No. 11-354919 (Prior Art 4), No. 2001-168519 (Prior Art 5), and No. 2001-36233 (Prior Art 6) and the like have been known.
In the prior art 1, for the lead-free solder, Snxe2x80x94Agxe2x80x94Bi system based solder or Snxe2x80x94Agxe2x80x94Bixe2x80x94Cu based solder alloy metal is mentioned. In the prior art 2, it is mentioned that solder of Snxe2x80x94Agxe2x80x94Bi system which leads as a lead-free solder is connected to an electrode the surface of which is treated with a layer of Snxe2x80x94Bi system. In the prior art 3, it is mentioned that electronic devices are reflow-soldered to both the 1st and 2nd surfaces of an organic board with lead-free solder which contains Sn as the main component, 0 to 65 mass % of Bi, 0.5 to 4.0 mass % of Ag, and total 0 to 3.0 mass % of Cu and/or In. In the prior art 4, it is mentioned that solder is cooled under an about 10 to 20xc2x0 C./s cooling velocity in a method by which electronic devices are connected to a circuit board with Bi containing lead-free solder. In the prior art 5, it is mentioned that, in a method by which an electronic device is surface connected and mounted on surface A of a board by means of a reflow-soldering, and then a lead of the electronic device inserted from the surface A side is flow-soldered on surface B of the board, the solder used for the reflow-soldering in the surface A side is a lead-free solder of such a composition as Sn (1.5 to 3.5 wt %), Ag (0.2 to 0.8 wt %), Cu (0 to 4 wt %), and In (0 to 2 wt %), and the solder used for the flow-soldering in the surface B is a lead-free solder of such a composition as Sn (0 to 3.5 wt %), Ag (0.2 to 0.8 wt %), and Cu. In the prior art 6, it is mentioned that, when flow-soldering by using a lead-free solder of an eutectic crystal composition the melting point of which is higher than that of a conventionally used Sn-37Pb, it is so intended that a temperature difference between an organic board and an electronic device main body will not increase while cooling the board after flow-soldering by using a heat conducting material in a space between a device main body and a board.
In this packaging method, as mentioned in the prior Art 6, in case of a flow-soldering in which molten solder jet was caused to come into contact with the lower surface of a circuit board, a case in which soldering was made under a temperature higher than that of a conventional method by using a lead-free solder of an eutectic crystal composition with a higher melting point than 183xc2x0 C. melting point of the Sn-37Pb (Unit: Mass %) or using a lead-free solder of a similar composition was considered.
However, the following points were not taken into considerations in anyone of the above-described prior arts 1 through 6.
In a packaging method, when preheating was made rapidly to shorten circuit board soldering time, or when a large volume of a component such as Pb which caused solder composition in a connected portion after the connection to considerably deviate from the eutectic crystal composition was contained in plating of an electrode, there was a possibility to cause the following three types of faulty soldering.
First of all, when flow-soldering a insertion mount device to a circuit board, a phenomenon called xe2x80x9clift offxe2x80x9d in which a solder in the insertion mount device connected position is peeled off from an electrode on the board due to a heat capacity difference of each insertion mount device occurs.
Secondary, the solder in an already connected surface mount device is molten again, causing the surface mount device to be peeled off.
Tertiary, because of a heat capacity of the already connected surface mount device, a component with a low melting point on the side near the board causes segregation in the solder in a connected portion when cooling the board after a flow-soldering, causing the reliability to drop down remarkably not only at the time of a connection by means of soldering but also after the connection.
Further, when conducting a rapid preheating to shorten circuit board soldering time, the above describing three types of faulty soldering are likely to occur.
Furthermore, in a conventional flow-soldering step, the soldering condition must be changed after all the flowing boards are carried out, and in particular, when manufacturing small batches of a variety of products, the loss of much time occurs.
It is an object of the present invention to provide a packaging method using lead-free solder capable of preventing a faulty soldering resulted from more and more uses of lead-free solder, and maintaining a high reliability of surface mount device connecting strength.
Another object of the present invention is to provide a packaging method using lead-free solder and flow-soldering system so as to reduce a loss of circuit board production time caused by producing small batches of a variety of products.
In order to accomplish the foregoing objects, the present invention is a packaging method using lead-free solder characterized by comprising a reflow-soldering step of soldering a surface mount device to an upper or lower surface of a circuit board with a lead-free solder paste; an inserting step of inserting a lead or terminal of a insertion mount device into a through hole drilled in the circuit board from an upper surface side of the circuit board; a flux applying step of applying flux to the circuit board after the lead or terminal of the insertion mount device is inserted into the through hole in the inserting step; a preheating step of preheating a lower surface of the circuit board after the flux is applied to the circuit board in the flux applying step; a flow-soldering step of cooling the upper surface of the circuit board the lower surface of which has been preheated in the preheating step, and flow-soldering the lead or terminal of the insertion mount device to the circuit board by applying lead-free solder jet to the lower surface of the circuit board; and a circuit board-double-side temperature control step of cooling or heating the upper surface of the circuit board, and cooling the lower surface of the circuit board, immediately after the soldering in the flow-soldering step.
Further, the present invention is a packaging method using lead-free solder characterized by comprising a reflow-soldering step of soldering a surface mount device to an upper or lower surface of a circuit board with a lead-free solder paste; an inserting step of inserting a lead or terminal of a insertion mount device into a through hole drilled in the circuit board from an upper surface side of the circuit board; a warp preventing jig attaching step of attaching a warp preventing jig made of metal such as aluminum or the like on the circuit board; a flux applying step of applying flux to the circuit board after the lead or terminal of the insertion mount device is inserted into the through hole in the inserting step; a preheating step of preheating a lower surface of the circuit board after the flux is applied to the circuit board in the flux applying step; a flow-soldering step of heating the upper surface of the circuit board the lower surface of which has been preheated in the preheating step, and flow-soldering the lead or terminal of the insertion mount device to the circuit board by applying lead-free solder jet to the lower surface of the circuit board; and a circuit board-double-side temperature control step of cooling or heating the upper surface of the circuit board, and cooling the lower surface of the circuit board, immediately after the soldering in the flow-soldering step.
Further, in the packaging method using lead-free solder, the present invention is characterized by further including a step in which a cover is attached to the portion where the surface mount device has already been attached to the lower surface of the circuit board as an additional step before the foregoing flux applying step.
Further, in the foregoing packaging method using lead-free solder, the present invention characterized in that the composition of the lead-free solder used in the foregoing flow-soldering step is an eutectic crystal composition or an equivalent eutectic crystal composition of an Snxe2x80x94Cu system, Snxe2x80x94Ag system, Snxe2x80x94Zn system, Snxe2x80x94Agxe2x80x94Cu system, Snxe2x80x94Agxe2x80x94Bi system or Snxe2x80x94Zuxe2x80x94Bi system. In particular, Sn-3Ag-xBi xe2x88x920.5Cu (0xe2x89xa6xxe2x89xa64, Unit: Mass %) is an eutectic crystal composition or a composition similar to the eutectic crystal composition of Snxe2x80x94Agxe2x80x94Cu system, in addition, has a melting point higher than the melting point (183xc2x0 C.) of the conventional Sn-37Pb, and can be used with a high connection reliability even under an extreme condition. Further, Sn-0.8Ag-57Bi is of an eutectic crystal composition or of a composition similar to eutectic crystal composition, and when used under a limited temperature, it can be used with a high connection reliability.
Further, the present invention is characterized in that the lead-free solder paste used in the foregoing reflow-soldering process is a solder alloy of an Snxe2x80x94Cu system, Snxe2x80x94Ag system, Snxe2x80x94Zn system, Snxe2x80x94Agxe2x80x94Cu system, Snxe2x80x94Znxe2x80x94Cu system, Snxe2x80x94Znxe2x80x94Bi system or Snxe2x80x94Agxe2x80x94Bi system, desirably, Sn-(1xcx9c4)Ag-(0xcx9c8)Bi-(0xcx9c1)Cu (Unit: Mass %).
Further, the present invention is characterized in that, the cooling of the upper surface of the circuit board in the flow-soldering step performs by blowing a liquid such as nitrogen or the like having a temperature not more than 50xc2x0 C. (desirably, in a temperature range of 20xc2x0 C. to 50xc2x0 C.).
Further, the present invention is characterized in that temperature of the lead-free solder jet applied to the lower surface of the circuit board in the flow-soldering step is within the range of 170xc2x0 C. to 260xc2x0 C.
Further, the present invention is characterized in, in the flow-soldering step, that the upper surface of the circuit board is heated to a temperature not more than 100xc2x0 C. (desirably, in a temperature range of about 150xc2x0 C. to about 200xc2x0 C.).
Further, the present invention characterizes in that, the heating of the upper surface in the flow-soldering step performs by blowing nitrogen in the temperature not less than 100xc2x0 C. The reason of that the temperature of the upper surface of the circuit board or the temperature of liquid such as nitrogen or the like using heating of the upper surface is not less than 100xc2x0 C., is based on that minimum temperature being effected on controlling the segregation of a lower melting point phase being formed with combination of lead-free solder and plating of an electrode in a connected portion is 100xc2x0 C.
Further, the present invention is a packaging structure (product) being characterized that is mounted a surface mount device and a insertion mount device on a circuit board by using the foregoing packaging method.
As described above, with the aforementioned composition, when preheating is made rapidly to shorten a circuit board soldering time in the case of that flow-soldering is made under a high temperature by using lead-free solder of an eutectic crystal composition or of a composition similar to the eutectic crystal composition the melting point of which is higher than 183xc2x0 C., or when electrode plating or the like contains a considerable amount of components such as lead or the like which cause the solder composition of a connected part to deviate considerably from an eutectic crystal composition after the connection, it can be made hard to occur a lift-off of a insertion mount device connected part, a remelting of solder in the surface mount device connected part, and a peel off of the surface mount device caused by segregation of the low melting point component in the solder.
Furthermore, the present invention is a flow-soldering system characterized by including a forth conveyor route provided with a carry-in unit which carries in a subjective circuit board which has been reflow-soldered using lead-free solder paste; a flux applying unit which applies flux to the circuit board carried in by the carry-in unit; a preheating unit which preheats the lower surface of the circuit board after applying flux to the circuit board at the flux applying unit; a flow-soldering unit which cools down the upper surface of the circuit board the lower surface of which was preheated by the preheating unit, or heats the upper surface of the circuit board with a warp preventing jig attached to the circuit board, and flow-solders by applying lead-free solder jet to the lower surface of the circuit board, and an adjusting unit to adjust temperatures of both surfaces of the circuit board which cools or heats the upper surface of the circuit board and further cools the lower surface of the circuit board, immediately after the soldering at the flow-soldering unit; and a back conveyor route along which the flow-soldered circuit board is returned from the unit to adjust temperature of both surfaces of the circuit board to a position near the carry-in unit on the forth conveyor route.
Further, the present invention is a flow-soldering system characterized by including a carry-in unit which carries in a subjective circuit board which has been reflow-soldered using lead-free solder paste; a flux applying unit which applies flux to the circuit board carried in by the carry-in unit; a preheating unit which preheats the lower surface of the circuit board after applying flux to the circuit board at the flux applying unit; a flow-soldering unit which cools down the upper surface of the circuit board the lower surface of which was preheated by the preheating unit,,or heats the upper surface of the circuit board with a warp preventing jig attached to the circuit board, and flow-solders by applying lead-free solder jet to the lower surface of the circuit; and an adjusting unit to adjust temperatures of both surfaces of the board which cools or heats the upper surface of the circuit board and cools the lower surface of the circuit board, immediately after the soldering at the flow-soldering unit, wherein the circuit board moving route comprises so as to be back planarly or spatially to the carry-in unit by way of the carry-in unit, the flux applying unit, the preheating unit, the flow-soldering unit and the unit to adjust temperatures of both surfaces of the board.
Further, the present invention is characterized in that the preheating is made in the preheating unit of the flow-soldering system also under the condition in which the circuit board is caused to stop.
Further, the present invention is characterized that further comprises a control memory which stores stop-time data being stopped the circuit board on the preheating unit, and a control unit which controls so as to stop the circuit board on the preheating unit.
Further, the present invention is a flow-soldering system characterized by comprising a holder-carrying conveyor which carries a circuit board holder mounted a circuit board from the preheating unit to the adjusting unit via the flow-soldering unit, and a station which can stop the circuit board holder on both of an entrance and an exit of the holder-carrying conveyor, whereby the flow-soldering system is able to flow-solder continuously for circuit boards which differs an optimum conveyor speed and an optimum preheating time by setting an optimum conveyor speed for carrying the circuit board from starting the entrance of the holder-carrying conveyor to finishing flow-soldering for the circuit board, and by setting an optimum preheating time on the preheating unit.
Further, the present invention is able to decrease time when must be discontinued the production of the circuit board by changing condition of flow-soldering when manufacturing small batches of a variety of products on base of controlling independently each of the conveyors which separates carrying conveyors for carrying the circuit board into a portion of carrying conveyor being in relation with flow-soldering and a portion of carrying conveyor being in relation of applying flux and including other portion.